In commonly owned British Pat. No. 1,472,464 there has been described a digital signal generator of this type comprising a memory which stores a number of sets of binary code words whose numerical values are representative of successive amplitude samples of a sine wave to be simulated, the various sets corresponding to different frequencies and/or amplitudes. A selected sequence of code words, read out from its store, is iteratively transmitted to a load to be tested for its performance, in terms of residual attenuation, frequency response, level dependence and cross-talk, without the need for a source of adjustable analog signals.